1. Field of the Invention
The present invention relates to a method and apparatus for measuring the parallelism of a pair of opposite surfaces of, for example, an anvil and a spindle of a micrometer, with a high accuracy.
2. Description of the Related Art
A micrometer is employed to measure a dimension of an object by interposing the object between the top surface of an anvil (a stationary surface) and the top surface of a spindle (a rotary surface) that can move linearly while rotating to measure a distance between both surfaces. It is required to measure the flatness and parallelism of such a pair of opposite surfaces with a high accuracy through, for example, a test method that has been known in the art to employ an optical parallel. A glass optical parallel used in this method has both surfaces or outwardly opposed reference surfaces that are optically lapped so as to be flat and in parallel with each other. The optical parallel is interposed between test surfaces of the micrometer to be tested or the pair of inwardly opposite surfaces to be measured, instead of the object in a usual measurement. When, interferograms (so called Newton fringes or rings) are created in contact portions between the reference surfaces of the optical parallel and the stationary and rotary test surfaces of the micrometer, they are observed visually in this method to evaluate the flatness and parallelism of the test surfaces.
This method, however, is disadvantageously difficult to execute an accurate measurement because the surface shape and parallelism is determined through the visual observation for the interferograms that appear on both surfaces of the optical parallel and therefore measures determined may reflect individual differences among operators. In addition, the conventional method requires some experience in the measurement work and thus is not always easy for anyone to perform it. This makes it difficult to reduce man-hours and costs.
The conventional method of measuring the flatness and parallelism based on Newton fringes is not applicable to the measurement through an image processing. It is also difficult to store measured data as detailed date in terms of the flatness and parallelism because a pass/fail determination is performed through the visual observation. Accordingly, the method is not possible to handle such a subject problem as an early detection of malfunctions and defects in manufacturing process and is not suitable for automating assembly and test processes.
Moreover, the test method with the conventional optical parallel, as described above, does not compute any obliquity of the rotary surface to the rotational axis (the spindle axis). Thus, it is not possible to determine the largest and smallest oblique angles between the stationary and rotary surfaces to compute the maximum and minimum of the difference between both surfaces.
From such the background, there is a requirement for a test method that is suitable for automation and capable of performing a stable measurement with a high accuracy.
In consideration of such the problems, the present invention has an object to provide an apparatus capable of measuring the parallelism of two opposite surfaces with a high accuracy without any experience and easily applicable for automation.
The present invention has another object to provide a method and apparatus, for measuring the parallelism of two opposite surfaces, capable of computing the largest and smallest oblique angles between the opposite surfaces, at least one of which can rotate.
In accordance with the present invention, an apparatus for measuring the parallelism of two opposite surfaces is provided. The apparatus comprises an interference optical system for leading parallel beam to a pair of opposed test surfaces and then leading, from the test surfaces via different optical paths, interference fringe images formed by radiation of the parallel beam to the test surfaces, respectively. It also comprises means for imaging the interference fringe images led via the different optical paths, respectively.
In the apparatus according to the present invention, leading parallel beam to a pair of opposed test surfaces and then leading interference fringe images from the test surfaces via different optical paths to means for imaging allows the interference fringe images at the test surfaces to be taken simultaneously and individually. Therefore, it is easy to obtain the flatness and parallelism of the test surfaces through the processing of results imaged from two interference fringe images.
In a preferred embodiment of the present invention, the interference optical system may include a measurement head having a pair of opposite reference surfaces formed thereon and defined with a highly accuracy parallelism and distance. The reference surfaces are interposed between the test surfaces. The reference surfaces each oppose to the respective test surfaces. The interference optical system may also include a lens system for collimating a light emitted from a light source into a parallel beam. It may further include a splitting optical system for splitting the parallel beam from the lens system into two optical paths, leading the two split parallel beam to the test surfaces via the reference surfaces and then leading said interference fringe images to the means for imaging. In this case, each of the interference fringe images is created through interference between a light reflected at each of the test surface and a light reflected at the corresponding reference surface opposing thereto.
In the interference optical system thus configured, two test surfaces and two corresponding reference surfaces create two interference fringe images, which can be imaged via two independent optical paths, respectively.
Preferably, for a highly accuracy measurement, the measurement head may be provided at the outside of at least one of the reference surfaces with a movable pressure plate for applying an appropriate measuring force onto the test surfaces.
Preferably, the apparatus of the present invention may fur comprise an arithmetic unit for computing the flatness and parallelism of the test surfaces from the interference fringe images taken by the means for imaging. The arithmetic unit may compute the flatness and parallelism of the test surfaces using at least three optical phase-shifted interference fringe images obtained from two sets of the means for imaging while altering a wavelength of the light from the light source in several stages. In this case, detailed data with respect to the test surfaces can be obtained through an arithmetic processing.
In accordance with the present invention, another apparatus for measuring the parallelism of two opposite surfaces is also provided. The apparatus comprises an interference optical system for leading parallel beam to a pair of opposed test surfaces and then leading, from the test surfaces via different optical paths, interference fringe images formed by radiation of the parallel beam to the test surfaces, respectively. In this case, at least one of the test surfaces is rotary relatively to the other about a rotational axis substantially along the opposing direction. The apparatus also comprises means for imaging the interference fringe images led via said different optical paths, respectively. The apparatus further comprises an arithmetic unit for computing the parallelism of the test surfaces from the interference fringe images taken by said means for imaging. The arithmetic unit measures an obliquity of a rotary test surface of the pair of test surfaces at a first position and at a second position rotated from the first position about the rotational axis by a predetermined angle. It then assumes from the obliquity of the rotary test surface at the first and second positions a cone or cones described by a normal vector of the rotary test surface. The arithmetic unit finally computes at least one of the largest and smallest angles between the pair of test surfaces from axes and vertical angles of the one or more cones assumed.
In one preferred embodiment of the present invention, the interference optical system includes a measurement head having a pair of opposite reference surfaces formed thereon and defined with a highly accuracy parallelism and distance. The reference surfaces are interposed between the test surfaces. The reference surfaces each oppose to the respective test surfaces. The interference optical system also includes a lens system for collimating a light emitted from a light source into a parallel beam. It further includes a splitting optical system for splitting the parallel beam from the lens system into two optical paths, leading the two split parallel beam to the test surfaces via the reference surfaces and then leading the interference fringe images to the means for imaging. In this case, each of the interference fringe images is created through interference between a light reflected at each of the test surfaces and a light reflected at the corresponding reference surface opposing thereto.
In accordance with the present invention, a method of measuring the parallelism of two opposite surfaces is further provided. The method comprises radiating parallel beam via reference surfaces to a pair of test surfaces. The test surfaces are opposed to each other. At least one of the test surfaces is rotary relatively to the other about a rotational axis substantially along the opposing direction. The method also comprises observing individually interference fringe images each obtained from interference between a light reflected at each of the test surfaces And a light reflected at the corresponding one of the reference surfaces to measure the parallelism of the test surfaces. The method further comprises the steps of: measuring an obliquity of a rotary test surface of the pair of test surfaces at a first position and at a second position rotated from the first position about the rotational axis by a predetermined angle; assuming a cone or cones described by a normal vector of the rotary test surface from the obliquity of the rotary test surface at the first and second positions; and computing at least one of the largest and smallest angles between the pair of test surfaces from axes and vertical angles of the one or more cones assumed.
In the method of present invention, an obliquity of a rotary test surface is measured at a first position and at a second position rotated from the first position by a predetermined angle. Then, a cone or cones described by a normal vector of the rotary test surface is/are assumed from the obliquity of the rotary test surface at each position. Therefore, it is possible to compute the largest and smallest angles from an angle between the axis of a normal vector of this assumed cone and a normal vector of the other test surface and a vertical angle of the cone.
The method of the present invention is applicable not only to the case where one of the pair of the test surfaces is rotary but also to the case where both of the test surfaces are rotary. If one of the pair of test surfaces is rotary and the other stationary, the method may comprise the steps of: measuring an obliquity of the other test surface and computing a normal vector of the other test surface from the obliquity measured; and computing at least one of the largest and smallest angles between the pair of test surfaces from an angle between an axis of a cone described by a normal vector of one test surface and a normal vector of the other test surface and a vertical angle of the cone.
If the pair of test surfaces are both rotary, the method may comprise the step of computing at least one of the largest and smallest angles between the pair of test surfaces from angles between axes of cones described by respective normal vectors of the test surfaces and vertical angles of the respective cones.
In a preferred embodiment of the present invention, the step of measuring an obliquity of a rotary test surface includes the steps of: obtaining a plurality of the interference fringe images with different phases through a plurality of measurements per one position and test surface; analyzing the plurality of interference fringe images to compute a height of each test surface acquired from the prey step; and computing an obliquity of a typical plane of the each test surface from the height of each test surface obtained from the preceding step.
The typical plane of the test surface may be computed using height data of each test surface through the least mean-square method. Alternatively, a plane that circumscribes the top portion or inscribes the bottom portion of the height data of each test surface may represent the typical plane. In measurement along with rotating the test surface, the apparatus of interferometer is necessary to be removed and rearranged. In this case, the standard test surface (the stationary surface) causes differences in its location to be measured. To solve this problem, the step of measuring an obliquity of a rotary test surface may comprise the steps of: obtaining a group of interference fringe images S1 of one of the pair of test surfaces at a first position and a group of interference fringe images R1 of the other at the first position; obtaining a group of interference fringe images S1xe2x80x2 of one of the pair of test surfaces at the first position and a group of interference fringe images R2 of the other at the second position; computing an amount of compensation required for matching a typical plane M1xe2x80x2 obtained from the group of interference fringe images S1xe2x80x2 with a typical plane M1 obtained from the group of interference fringe images S1; and compensating a typical plane obtained from the group of interference fringe images R2 with the amount of compensation. From this alternative, even if the standard test surface (the stationary surface) causes differences in its location to be measured, the differences can be compensated.
The amount of compensation may include a rotational axial position and rotational angle for matching the normal vector of the typical plane M1xe2x80x2 with the normal vector of the typical plane M1, which is also used as compensation data to ensure the compensation.